The present invention generally relates to voltage regulation, and particularly relates to improving the transient response of voltage regulators, including those regulators having output voltage droop control.
The term “voltage regulator” generally connotes a device or circuit that provides a regulated output voltage to a load. Such devices are sometimes also referred to as “power controllers.” Neither term conveys the complexity and sophistication of contemporary voltage regulators, which find use in a wide range of demanding applications.
Providing the operating voltage to a modern, high-performance microprocessor core stands as a prime example of such applications. Microprocessors tax the abilities of even the best voltage regulators to maintain accurate output voltage regulation. For example, at least some processors require different supply voltages corresponding to different modes of operation. Lower supply voltages commonly correspond to lower power (and lower performance) modes of operation, while higher supply voltages generally are required for higher or maximum performance modes of operation. Thus, a voltage regulator intended for operation as a microprocessor core voltage regulator may need to regulate accurately to any one of several specified output voltages.
Moreover, microprocessor current requirements generally have a wide dynamic range. For example, a given microprocessor might draw 500 mA to 1 A while operating in a low-power mode, but might have peak current draws over 40 A in higher performance modes. Worse still, the changes in current draw, i.e., the load change, may be so rapid that it is difficult for the voltage regulator to prevent voltage undershoot or overshoot on the regulated voltage output.
Indeed, specialized techniques have developed to combat the undershoot/overshoot problems. For example, some voltage regulators incorporate a droop control mechanism that “droops” the output voltage as a function of current load. As a simple example, the nominal operating voltage of a given microprocessor might be specified as 1.8 V, but the actual permissible range of supply voltage may be 1.5 V to 2.2 V. A voltage regulator with droop control might be configured to regulate the output voltage to a value in the higher end of the range under light current loads, and then regulate down toward the lower end of the permissible supply voltage range as the current load increases.
Thus, the voltage regulator uses load current feedback, or some related variable, as a control input for actively positioning the regulated output voltage. Positioning the output voltage at a relatively high level within the permissible supply voltage range under light load conditions tends to prevent voltage undershoot arising from step change increases in load current. Similarly, positioning the output voltage at a relatively low level within the permissible supply voltage range under heavy load conditions tends to prevent voltage overshoot arising from step change decreases in load current.
The need to sense load current stands as one potentially undesirable aspect of active voltage positioning. For example, some voltage regulators require the addition of a current sense resistor to their outputs as a means of providing feedback to their droop control circuits. Resistance-based current sensing can be undesirable because it requires the addition of at least one resistor to the Bill of Materials (BOM). Oftentimes that resistor is large and expensive, given the high currents that are involved. Further, resistance-based current sensing unavoidably wastes some amount of power, which is always frowned on in power-sensitive mobile processor applications.
Thus, designers have developed alternative, essentially “lossless” approaches to sensing load or load-related currents. For example, in switch-mode voltage regulators, voltage droop control may be based on sensing switch currents in the inductor-switching transistors, or current sensing may exploit the dc winding resistance (DCR) of the switched inductor. These lossless approaches to sensing load current potentially have sensing time constants that are large with respect to the voltage regulator's switching period. As such, the relatively slow response of the lossless current sensing can affect the overall load transient response of the regulator.